The present invention relates to an optical switching apparatus and an optical transmission system using the same, and a method of setting up for an optical signal route in the optical switching apparatus.
In order to support rapid increase in data traffic represented by the Internet and also rapid increase in demands for multimedia communications for images/sounds/data, transmission lines and telecommunication nodes constituting communication networks are promoted to be higher speed operation and larger capacity operation, and introduction of optical communication apparatuses using optical fibers and optical signals is also promoted.
Further, optical switching apparatuses called as an optical cross connect (hereinafter referred to as “OXC”) and an optical add-drop multiplexing apparatus (hereinafter referred to as “OADM”) with which optical signals are subjected to the switching processing such as transmission route switching/line switching, etc. without converting the optical signals to electrical signals are considered to be practically used in place of a conventional communication apparatus for temporarily converting optical signals to electrical signals and then processing the electrical signals.
When an optical switching apparatus as described above is practically used, it is important to provide an apparatus which is enhanced not only in basic performance of suppressing loss of optical signals power, but also in reliability, availability and serviceability performance (hereinafter referred to as “RAS”). For example, there is considered such a construction that the performance of signals to be transported are monitored at a proper position, or such a construction that a redundant structure (for example, doubling) is adopted for a part of the apparatus and the route of signals is properly switched when the apparatus breaks down or the performance thereof is deteriorated.
The RAS enhancing means described above is practically used in conventional transmission apparatus and digital exchange in which data are processed in the form of electrical signals. For example, in the conventional transmission apparatus using the electrical signal processing, it is little necessary to consider the time which is required to switch the route of signals by an electrical switch. Therefore, it is relatively easy to perform the switching operation with neither data lack nor data duplication (power-uninterruption) and a power-uninterruption switching function using a buffering technique based on a memory or the like is known.
As described above, optical signals are directly processed in OXC and OADM, and a mechanical type optical switch which is low insertion loss is frequently used as an optical switch serving as a main part for setting a route. However, it is known that the mechanical type optical switch has a lower optical switching speed as compared with the transmission speed of optical signals to be transmitted therethrough and several milli-seconds are needed to change the route. If the same signal route switching operation as the conventional apparatus based on the electrical signal processing is performed in OXC or OADM which directly processes the optical signals, there would occur a power-interruption that no optical signal is output during the route switching operation for the optical signals because of the low switching speed of the optical switch as described above and thus the optical signals corresponding to an extremely large amount of data are not passed through the optical switch concerned and thus lost. Accordingly, it is required to implement an optical signal switching apparatus having excellent RAS in consideration of the power-interruption inherent to the apparatus due to the optical signal processing as described above.
In OXC and OADM, a monitoring portion for monitoring various monitoring items such as (1) power deterioration/interruption of optical signals, (2) the synchronous state of operating clocks, (3) the synchronous state of optical signal frames, (4) the bit error rate of optical signals (hereinafter may be referred to as “BER”), etc. is provided in order to keep the performance of the optical signals to be processed. When the monitoring portion detects a failure, the route of the optical signals may be switched to a normal one to thereby enhance RAS.
However, according to the apparatus having such a failure monitoring function, when the route setting is changed to enhance RAS, an unnecessary optical signal power interruption alarm, a bit error rate abnormality alarm, an out-of-sync alarm or the like may be detected in accordance with the switching speed of the optical switch, the setup position of the monitoring portion and the monitoring method.
Further, in a transmission system such as SONET/SDH or the like, monitoring control information is received/transmitted separately from main signals between respective apparatuses constituting a network. Therefore, there may occur such a situation that an undesired alarm detected when the route switching is carried out as described above is informed as an error alarm to an apparatus located at the downstream side in the travel direction of optical signals or an apparatus for monitoring and managing failures of the network even though the switching operation of the optical switch is normal.
Still further, the monitoring portion checks normality of a new route after the route switching operation is carried out when some failure occurs, and also monitors a restoration status of the old route under failure. Accordingly, if the monitoring is not carried out in consideration of the time required for the switching operation of the optical switch and the operation time of the failure monitoring function, an error alarm may be informed to an apparatus at the downstream side or a monitoring apparatus.
In such a case, even though a newly-switched route is normal, the switching operation is induced to switch to another route again, so that the operations of individual apparatuses such as OXC, OADM, etc. and the operation of a communication system (network) using these apparatuses may be made unstable.